Ultraviolet (“UV”) imaging is used for industrial inspection processes. UV light interacts with materials in a manner that enables features and characteristics to be observed that are difficult to detect by other methods. For example, UV light is strongly absorbed by many materials, making it possible to visualize the surface of an object without the light penetrating into the interior.
The UV band spans the range of wavelengths between 10 nm (where the x-ray band begins) to the edge of human visual sensitivity at 400 nm. There are two main classes of industrial UV imaging applications that each use a different band of the UV spectrum. The band of the spectrum between 300 nm and 400 nm is commonly known as the near UV band. It is divided into the UV-A and UV-B sub-bands. The band of the spectrum between 100 nm and 300 nm, which includes the UV-C band, is known as the deep UV (DUV) band.
UV imaging has advantages over visible light. Because of its short wavelength, UV light tends to be scattered by surface features that are not apparent at longer wavelengths. Thus, smaller features can be resolved or detected by scattered UV light. For example, UV imaging can be used to detect scratches or surface imperfections that are not apparent in an image formed using visible light.
DUV applications are growing in importance for nanotechnology and lithography. In the semiconductor industry, inspection of photomasks or wafers with fine lines and features may need to find defects that are submicron in size. Visible light may be unable to resolve features that are so small. Confocal microscopes operating in the DUV band can be used to image these features with much greater clarity than in the visible band. Detection of these defects early in the production process can greatly improve yields and reduce waste.
Multilayer coatings made of layers with different optical properties have proven effective for various DUV applications. However, many multilayer coatings exhibit large residual stress. This is particularly true in DUV where fluoride materials or other materials are used. Some of this stress is extrinsic and caused by a mismatch between the coefficient of thermal expansion (CTE) of the substrate and the multilayer coating. A CTE refers to a material's change in size per change in temperature at a constant pressure. As a material is heated, kinetic energy increases and molecules usually maintain a greater average separation. Mismatches in CTE can lead to fractures or other defects in a multilayer coating due to temperature changes. Therefore, what is needed is an improved multilayer coating.